3.3. CFD analysis
Differences were observed between the culture results of the rotary
bioreactor and reciprocal bioreactor. The differences seemed to be
influenced by the physical properties of each bioreactor. CFD
simulations were used to clarify the relationship between culture
results and physical action. In general, low shear stress is required
for animal cell culture; however, our study showed uniform floating and
dispersal of cells, and appropriate stimulus to cells to also be
important.
Fig. 4A shows the vector of rotary bioreactor with EE, and Fig. 4B–D
show the vector at the impeller positions “top dead point”, “passing
through middle point”, and “bottom dead point”, respectively, of a
reciprocal bioreactor. In the rotary bioreactor, weak flow zone exists
near the center, under the impeller; however, in the reciprocal
bioreactor, flow pattern spreads throughout the entire culture vessel.
Such flow, as in the reciprocal bioreactor, is very important for cell
culture. Fig. 4E shows the shear stress in rotary bioreactor, and Fig.
4F–H show the shear stress at the impeller positions, as shown above.
Although the most abundant shear stress (around 1 Pa, Fig. 5) was
similar to each other, characteristic distribution curve possessing two
peaks was drawn in reciprocal bioreactor. Most importantly, the stress
generated by reciprocal motion was not continuous while that in rotary
bioreactor was generated continuously. Industrial production using
animal cell culture starts by switching to protein production when cell
growth reaches a stationary state. Therefore, the damage that cells
undergo while facing prolonged and continuous shear stress in rotary
bioreactor is considered to be more serious than that caused by
discontinuous shear stress in reciprocal bioreactor. Moreover, it is
also undesirable for the cell environment, since shear stress creates in
a non-uniform condition in the bioreactor.
As shown in Table 1, both maximum and average shear stresses in rotary
bioreactor were found to be higher than that in the reciprocal
bioreactor. In addition, the simulation showed a large variation in the
shear stress and shear rate in the rotary bioreactor. Cells in rotary
bioreactor are continuously exposed to such a stressful turbulence, and
accumulate the damage to eventually affect their physiological
condition. Thus, the physical characteristics of the reactor are
reflected in the culture results.